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 Steel in the sound concrete
 

Before being placed in a formwork, a reinforcement is rusted, because it was initially exposed to atmosphere.

armature en acier


When a freshly-mixed concrete is placed around this steel, the mixing water penetrates through the rust pores, where it gradually forms hydrated calcium ferrite (4.CaO. Fe2 O3 13H2 O). Moreover, this water reacts with steel and forms on it a thin layer of iron and calcium hydroxides, respectively [Fe(OH)2] and [Ca(OH)2].

All these products in the vicinity of steel raise the pH of concrete pore solution, up to about 13. It should be noted that in contact with an initial rust, cement hydration of is disturbed : a transition zone is locally formed, and concrete is more homogeneous, far from this zone.

So, the concrete mixing water makes it possible to form around steel, some products, which protect it by passivation. More precisely, under atmospherically induced rust, reinforcement is covered with a thin protective layer of white products, containing ferrite and of hydroxide of calcium.

Such a protection vanishes, if the pore solution disappeared (e.g. when large cracks reach reinforcements) or does not correspond any more to sound concrete.

armature en acier

Partial depassivation of a reinforcement : Protective products are white

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 Stages of steel corrosion
   

Corrosion with rusting of reinforcement in concrete comprises two stages. In a first stage (or step), the aggressive elements, such as chloride or carbon dioxide (CO2) (Cl-), present in the surrounding medium, penetrate in concrete. This is the initiation stage. The second stage is propagation which starts, when these aggressive bodies are in rather high concentrations at the reinforcement level. This corresponds to the rust growth, which can break concrete cover.

These stages are described as follows.
- a first stage involves the transfers of aggressive agents (mainly carbon dioxide and chloride) of water and of oxygen, inducing the corrosion initiation (depassivation of reinforcement),
- a stage of corrosion growth, leading to concrete damage, to spalling, cracks, etc. This stage starts when the contents of aggressive agents are high enough closed to reinforcing steel.

So, to describe steel corrosion in concrete, it is advisable to define, on the one hand, the
penetration of the aggressive agents through concrete and, on the other hand, the conditions of depassivation of reinforcement, then the dissolution rate of metal and the rust growth.


Stages of corrosion with rusting

(1) initiation

(2) propagation starting from date (d)

(3) concrete cracking

Stade (1) initiation
Stade (2) propagation
Stade (3) cracking
 

No visible damage

Damage hardly detected
Damage visible
on concrete surface

See the flash animation about damage stages


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 Corrosion process of tendons in prestressed concrete
   

In a prestressed structure, concrete is previously stressed under compression by using steel tendons, so that any further tensioning on service results in a very low tension strain in the material.

When concrete is prestressed by pre-tensioning, tendons are in contact with concrete (Fig. 1.A). In post-tensioned structures, tendons are in ducts filled with cement grout or an organic product (wax, grease) (Fig 1B, 1C). So, possible corrosion is due to incorrect filling of the ducts.

Prestressing tendons can corrode either by the usual process, including dissolution and rusting, or by cracking, when they are tensioned (stress corrosion). In the first case, cracks in steel are initiated at the roots of small pits. In the second case, cracks are due to hydrogen embrittlement, mostly without visible dissolution or rust.

After initiation, a crack grows slowly until the tendon breaks. The lifetime depends on various parameters: steel grade, type of environment, applied stress level. So, only durable tendons are chosen for real structures, after adequate stress corrosion tests.






Crack on the surface of a wire broken by stress corrosion

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 Deterioration of concrete cover
   

A concrete cover around reinforcement can be deteriorated by the surrounding environment, because of various origins:

    physical causes: freezing, etc;
    mechanical causes: concrete can crack under an excessive loading,
    chemical causes, for example, due to bodies (gas or ions) contained in the environment.

As a rule, reinforced concrete structures are in contact with atmosphere, with water (river, sea, etc.) or with ground. These environments are more or less polluted and contain some bodies (gas or liquid) which can enter concrete and modify its characteristics, particularly the chemical composition of its pore solution.

The most frequent aggressive agents are pure water, chlorides dissolved in water and carbon dioxide (CO2) in atmospheres.

Pure water can wash out (leaching) concrete by dissolving some cement components and increase concrete porosity.

Chloride are highly soluble in water. So, these ions are dissolved in water, and penetrate with it in concrete (chlorides penetration), either by the wetting of a non saturated concrete (convection), or by diffusion because the chloride content is higher in the environment than in the original concrete (concentration gradient). The main part of chlorides coming from outside remain as dissolved ions, in the concrete pore solution. But they can also react with some components of the material (chemical reaction or adsorption).

The carbon dioxide (CO2) is a gas in the atmosphere. It can be dissolved by the concrete pore solution, and react with some calcium compounds to form carbonates (carbonation). So, the pH is about 9 for the pore solution of a concrete deteriorated by carbonation.

The penetration of carbon dioxide in concrete is a diffusion process It is fast when the concrete is rather dry. But the carbonation reaction does not occur if no pore solution is present in concrete. This is the reason why, the most favourable conditions to the penetration of carbon dioxide correspond to a relative humidity of about 65%.


Practical consequences : To slow down the penetration of the aggressive agents, it is necessary to formulate and to make concrete, so that its porosity is low and the diffusion coefficients of these agents are low.

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 Initiation of reinforcement corrosion
   

The reinforcement corrosion is initiated, when the products formed on steel do not protect them any more (depassivation), because they become more porous. So, a first criterion of corrosion initiation corresponds to the change of the nature of these products. This process includes intermediate stages which give more or less stable products, " green rusts ". A criterion for corrosion initiation, which is more operational, corresponds to a significant modification of dissolution metal rate (change of the corrosion activity). To apply this criterion, it is necessary to follow up corrosion rate.

 

Formation of corrosion products of iron, in a concrete sound or " polluted " by chlorides or carbonates

In practice, carbon dioxide (CO2) initiates reinforcement corrosion, when the concrete cover in contact with this steel is carbonated and rather wet (even in a non permanent way). Chlorides which induce metal corrosion, are ions dissolved in the pore solution ("free chlorides"). Their critical content depends mainly on concrete pH and oxygen content.

Practical consequences : For a given concrete and a environment, corrosion initiates later, when the cover abovef steel is thicker and more compact.

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 Corrosion growth
   

Some iron products are protective when they remain stable and are thin, e.g. ferrous hydroxide Fe(OH)2 " green rust " , magnetite. On the other hand, products which do not stop corrosion grow with time.

When a reinforcement corrodes, it undergoes a more or less localised dissolution, and it is covered with unstable corrosion products (traditional rust of reddish colour). In a rather porous and wet concrete, these products migrate through the cover and can stain the concrete surface. But, in the more traditional case, for a relatively dry concrete, the corrosion products swell by highly deforming the cover and, under a effect, can crack the concrete or make spalling.

The crack orientation depends on the geometry of reinforced concrete (steel diameter, cover thickness, plane or angular concrete surface).



The reduction of steel cross area and the simultaneous rust swelling induce a more or less significant decrease of the bonding between steel and concrete.

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 References
    Documents in French

    " La corrosion et la protection des aciers dans le béton " par A. RAHARINAIVO, G. ARLIGUIE, T.       CHAUSSADENT, G. GRIMALDI, V. POLLET, G. TACHÉ, Ed. Presses de l'École Nationale des Ponts       et Chaussées, Paris, 1998

   "Corrosion des armatures dans les bétons " «Partie 1 Mécanismes » V. L’HOSTIS, A. RAHARINAIVO, G. TACHÉ, Techniques de l’Ingénieur, Paris, Article C 407 (2009)


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